Underfoot Combat vs 5-foot Steps

[Iron Angel]

My Goblin Fighter just hit 9, so my level plan is to take Underfoot Combat. We're starting to get to the point where the big threats are Large+ things and this feels like a good time to start investing in combating those, and as a Small character, this seems like a good way to go about it.

The problem is our DM isn't happy unless someone is dying and he tends to make monsters master tacticians (We fought against orcs who deliberately went around people to avoid AOOs last session) so I just know that unless I can get in the middle square of something Huge+ he's just going to use 5-foot steps to deny me every bonus Underfoot Combat gives and basically give me a wasted feat.

So what I need is a way to prevent 5-foot steps. I know you can't make 5-foot steps through Difficult Terrain but I can't find anything that would let me just conjure up some difficult terrain. I thought about buying some Caltrops but those don't count as difficult terrain and you can make a 5-foot step in those (Please, please correct me if I am wrong). I can't find anything else either.

Is there any way to just remove the ability of a Large opponent for making 5-foot steps? I thought about having the Ranger cast Spiked Growth or Entangle on the enemies but either A: I have to enter the entangled area afterwards, which means no tumbling to get under the enemy, which means AOO, or they have to cast it with me already there, which means I potentially have to make the save as well and possibly fail it.

Please help. Our party is one Fighter/Exotic Weapon Master (me), one Fighter/Sorcerer (one rank)/Dragon Disciple, a Sorcerer (who is the DM's character and knows only damage spells anyway), a Paladin, a Rogue, and a Ranger/Druid.

[heavyfuel]

3 levels in Knight (PHB II) makes your threatened are Difficult Terrain.

Entangle (Ranger and Druid 1) also makes for Difficult Terrain, as well as Web (Sor/Wiz 2), both last long enough for a whole combat and then some.

I could be wrong, so if I am, somebody correct me, but forcing Balance checks will make people unable to take 5ft steps. You can do that with Grease (Sor/Wiz and Psion 1) as well as Marbles that costs 2sp (A&EG).

Giantbane (CWar) feat can keep move you with the creature (and there's no RAW forbidding usage in conjunction with Underfoot Combat or Confound Big Folk)

Edit: Caltrops aren't difficult terrain. Check to see if the Sorcerer can get Selective Spell Metamagic (Shining South). It's one of the best MM feats out there, though it's campaign specific and your DM may forbid it.

[Iron Angel]

I should note that our DM is fairly new so a lot of books are clean off the table to keep things simple. We're using core, the Complete series of books (Complete Divine, Complete Scoundrel, etc), and the Races series of books (Races of Stone, Races of the Wild, etc). Since I'm chaotic, Knight is right out. As for Giantbane I don't think you can use Climb Aboard and the benefits of Underfoot Combat at the same time (It would be pretty sick if you could though, +8 AC total against the thing you're fighting? Yes please). Maybe by RAW since you still fulfill the requirements to take the advantages but its rather silly to be both climbing on your opponent's back AND be running around their legs and is a pretty clear breach of RAI, at least to me.

[heavyfuel]

I should note that our DM is fairly new so a lot of books are clean off the table to keep things simple. We're using core, the Complete series of books (Complete Divine, Complete Scoundrel, etc), and the Races series of books (Races of Stone, Races of the Wild, etc). Since I'm chaotic, Knight is right out. As for Giantbane I don't think you can use Climb Aboard and the benefits of Underfoot Combat at the same time (It would be pretty sick if you could though, +8 AC total against the thing you're fighting? Yes please). Maybe by RAW since you still fulfill the requirements to take the advantages but its rather silly to be both climbing on your opponent's back AND be running around their legs and is a pretty clear breach of RAI, at least to me.

Is it silly? Maybe. But far from what's been done before. If you've ever played Dragons Dogma, then you probably climbed your way from a Cyclops belly down to his groin area, stabbed him a few times there just for laughs, and proceeded to climb between his legs until you reach the back and eventually the head.

It's seriously up to how you fluff it. Of course, your DM needs to OK this.

After checking the Rules Compendium, I don't think forcing balance checks counts as a "hampered movement" so it doesn't work by RAW, but honestly, that would make Grease and Marbles pretty useless since the enemy could always take a 5-foot step to get out of the area. Apparently, by RAW, even a prone creature can take a 5ft step...

If your DM allows that forcing balance checks and being prone means no 5ft step, then having the Sorcerer cast Grease will work wonders, making them balance for round level on a relatively small area, so you should be able to avoid it. Sculp Spell (CArc) goes hand in hand with Grease.